Cell Membrane

Why can nonpolar molecules pass through the phospholipid bilayer easily?

Nonpolar molecules are compatible with the nonpolar, hydrophobic fatty acid tails in the middle of the membrane, so they can move through the hydrophobic interior relatively easily.

Why can't most polar molecules pass through the phospholipid bilayer easily?

Polar molecules can interact with the polar heads, but they cannot interact favorably with the nonpolar, hydrophobic tails in the middle of the membrane. They therefore usually need a transport protein to cross.

What does it mean that the cell membrane is selectively permeable?

The cell membrane allows some substances to cross more easily than others. Small nonpolar molecules can cross relatively easily, while many polar molecules and ions cannot cross the lipid bilayer without help from membrane proteins.

How do protein channels make the cell membrane selectively permeable?

Specific protein channels provide routes through the membrane for specific substances. This allows the membrane to control which materials can enter or leave the cell.

Why are proteins the perfect molecules to build structures in the cell membrane?

Proteins contain amino acids with both polar and nonpolar R groups. This allows different regions of a protein to interact with both the hydrophobic membrane interior and the watery environments on either side of the membrane.

How do nonpolar amino acids anchor a transmembrane protein in the membrane?

The nonpolar amino acids have hydrophobic R groups that interact with the hydrophobic fatty acid tails inside the membrane. This helps anchor the protein within the phospholipid bilayer.

What do nonpolar amino acids have in common, and where are they usually found in proteins?

Nonpolar amino acids have nonpolar R groups. Because they are hydrophobic, they tend to be found away from water, often buried inside proteins where they can interact with other nonpolar groups.

What is the difference between polar and charged amino acids?

Charged amino acids are a type of polar amino acid because they have a full positive or negative charge. However, not all polar amino acids are charged; some are polar but have no full charge.

How can you tell whether an amino acid R group is polar or nonpolar?

Look at the functional groups in the R group. R groups containing polar atoms such as oxygen or nitrogen are often polar, while R groups made mostly of carbon and hydrogen are generally nonpolar. A full positive or negative charge means the R group is charged and therefore polar.

What is the cytosol?

The cytosol is the watery fluid inside the cell. It provides the watery environment that the polar regions of membrane proteins can interact with on the inside of the cell.

How are amino acids arranged in a membrane protein?

Within the membrane, nonpolar amino acids are hydrophobic and interact with the nonpolar fatty acid tails, helping anchor the protein. On the outer surface, polar amino acids are hydrophilic and extend into the extracellular fluid, while polar regions on the inner surface extend into the cytosol.

What is enzyme activity as a function of a membrane protein?

A membrane protein can function as an enzyme by speeding up a specific chemical reaction. The part of the protein responsible for the reaction, such as its active site, is positioned where it can interact with the appropriate molecules.

What is a cell surface receptor and what does it do?

A cell surface receptor is a membrane protein that binds to a specific signaling molecule outside the cell. When the signal binds, the receptor helps transmit that information into the cell, causing the cell to respond. It is like a doorbell that detects a signal outside and sends a message inside.

What is a cell surface identity marker, and how can glycoproteins and glycolipids act as identity markers?

Cell surface identity markers act like ID tags that allow cells to recognize and distinguish one another. Glycoproteins have carbohydrate chains attached to proteins, while glycolipids have carbohydrate chains attached to lipids. These carbohydrate chains extend from the cell surface and can function as identity markers.

How do membrane proteins function in cell adhesion?

Membrane proteins on one cell can bind to proteins on a neighboring cell, helping the cells stick together. This allows cells to form and maintain tissues.

How can membrane proteins attach to the cytoskeleton?

A peripheral protein on the inner surface of the membrane can attach to cytoskeleton filaments inside the cell. This helps connect the cell membrane to the cytoskeleton and helps support the cell's structure.

What are the two main classes of membrane proteins?

Peripheral proteins are loosely bound to the surface of the membrane and do not penetrate the lipid bilayer. Integral proteins penetrate the lipid bilayer and are embedded within it; some integral proteins cross the entire membrane.

What is the difference between a peripheral protein and an integral protein?

Peripheral proteins are loosely attached to the surface of the membrane and do not penetrate the lipid bilayer. Integral proteins are embedded within the lipid bilayer and penetrate into the membrane.

What is the difference between an integral protein and a transmembrane protein?

Integral proteins are embedded in the lipid bilayer and may penetrate partway or all the way through it. A transmembrane protein is a type of integral protein that crosses the entire lipid bilayer.

What does transmembrane mean? What are its functions?

Transmembrane describes a protein that crosses the entire phospholipid bilayer. It describes the protein's structure, not its function. A transmembrane protein can function as a channel, receptor, enzyme, or in other membrane roles Cell-cell recognition.

What is the fluid mosaic model of the cell membrane?

The fluid mosaic model, proposed by S. J. Singer and G. Nicolson in 1972, describes the cell membrane as a fluid phospholipid bilayer with a mosaic of proteins and other molecules embedded within it.

Why must the cell membrane remain fluid, and how do saturated and unsaturated fatty acids affect fluidity?

The membrane must remain fluid and flexible so its components can move and the membrane can function properly. Unsaturated fatty acid tails contain kinks that prevent tight packing, increasing fluidity. Saturated tails are straight and pack more tightly, making the membrane less fluid.

Why do cold-adapted organisms increase their percentage of unsaturated fatty acids?

Cold temperatures make membranes less fluid and more rigid. Cold-adapted organisms increase unsaturated fatty acids because their kinked tails prevent tight packing, helping the membrane remain sufficiently fluid in colder conditions.

What role do membrane carbohydrates play in cell recognition?

Membrane carbohydrates act as cell surface ID markers, allowing cells to recognize and distinguish one another. This is important during organ and tissue development because cells need to recognize and interact with other cells, and it is important for the immune system to distinguish the body's own cells from foreign cells.